Hi and welcome to the forum.
From the question I assume the CPAP will run on 12V DC or 110V AC?
If yes, which one directly powers the unit, that is does the pump run on 12V that is converted from 110V or vice versa.
Whichever runs it directly avoiding conversion from whatever you plug into on the boat will be the most efficient.
The other question. I'm not familiar with a CPAP and how much power it uses. Look on the unit and there will be a label somewhere that will give the power usage. Might say something like 2 amps at 110V AC or 250 Watts or 1.5 amps at 12V DC. With that you'll know how much power the CPAP will need. Then you need to know what power capacity the boat has. Is there a generator
on board? How many and what size batteries? Conch Charters should be able to tell you this.
Since you mentioned it in your post maybe you can tell me. Why do many first posters ask repliers to be gentle.? Sorry I don't get it. I'm pretty sure if some less than polite person posted a less than polite reply it would not cause actual pain. You know, sticks and stones and all that. Besides, the cardinal rule
of the forum is "be nice".